1. Field of the Invention
The present invention relates to a compact mobile phone, and more particularly a retractable antenna unit provided in a mobile phone operating in a low frequency band.
2. Description of the related art
A mobile phone, such as a cellular phone, is typically provided with a whip or helical antenna, whose length is .lambda./4 or 3.lambda./4, where .lambda. is the wavelength of the received signal in accordance with the operating frequency. If a whip antenna is used, it must be in a extended position to maximize efficiency during calls, making it less compact and therefore, inconvenient for the user to carry. On the other hand, a helical antenna has a short physical length, and hence does not bring about such inconvenience. However, the efficiency of a helical antenna is low when compared to that of a whip antenna, due to its short physical height. Therefore, due to the helical antenna's degraded performance when compared to the whip antenna, it is desirable to have an antenna with a relatively longer physical height, such as the whip antenna.
In this view, a retractable antenna has been developed which combines a whip and helical antenna to realize the advantages of both types of antenna. Such an antenna is well disclosed in U.S. Pat. No. 5,479,178. Referring to this patent, when a mobile phone stands by to receive a call, the whip antenna may be retracted into the casing of the mobile phone, utilizing only the helical antenna for receiving incoming calls. When communication is established, the whip antenna may be completely extended from the casing to improve communication quality for the duration of the call.
However, such a retractable antenna would only be sufficient for the end if the body of the mobile phone is long enough to fully receive the retracted length of the whip antenna. Moreover, there is a trend toward making mobile phones more compact, especially with the current state of development of VLSI (very large system integration) technology, making space in a mobile phone's casing a premium. Further, when retracting the whip antenna into the casing, it must be decoupled from the wave feeding point. However, if the space for retracting the whip antenna into the casing is lacking, it becomes impractical to achieve complete decoupling.
Consequently, space is one of the key factors limiting the design of an antenna. Therefore, a need exists for a retractable antenna unit combining a whip antenna with a helical antenna, wherein the whip antenna of length .lambda./3 or 3.lambda./4 can be completely
retracted into a compact mobile phone while the helical antenna remains projected outside of the casing.